1. Field of the Invention
The present invention is directed to a method for directly electroplating metals onto dielectric base materials. More particularly, the present invention is directed to a novel method for directly electroplating metals onto the surfaces of non-conducting or dielectric substrates without the need for preliminary electroless plating, conversion coatings, solution additives, or conductive clips to initiate propagation of plating metal. The method of the present invention is applicable to multi-layer laminated circuit boards and copper clad substrates and is particularly well suited to pattern plating processes, non-clad substrates, and molded circuitry. Moreover, in addition to allowing increased circuit density through the increase of through hole aspect ratios the method of the present invention and the plated substrates so produced eliminate substantial plating and manufacturing costs.
2. Description of the Prior Art
Numerous methods for the metallic plating of dielectric surfaces are known in the art and have proven to be useful in the production of printed electrical circuit boards. Generally speaking, the metallic plating of dielectric surfaces is accomplished by first making the surface catalytically receptive to electrolessly formed metal deposits followed by electrodeposition of a plating metal over the electrolessly formed conductive metal deposits. Though effective at producing conductive metal layers upon non-conducting substrates, such multi-step electroless plating methods are expensive and limited in their application due to the chemical susceptibility of the electroless layer and the need for stringent process control. Further limitations exist on the process of electroless plating because it requires the use of extremely hazardous and toxic materials.
Efforts at overcoming these limitations and at reducing the associated expenses and reducing the health risks through the elimination of the electroless plating step have met with only partial success. Generally, these alternative methodologies substitute additional steps such as coating the substrate with conductive materials in place of the electroless plating. Though direct plating of dielectric materials through the use of graphite coatings or conductive metal powder coatings in place of the electrolessly deposited metal layers has been achieved these methods are equally cumbersome and carry their own disadvantages.
More specifically, early electroless plating catalyst systems consisting of palladium chloride and tin chloride in acidic solutions were developed in the late 1940's and early 1950's. For example, U.S. Pat. No. 2,702,253 (Bergstrom) disclosed a two step procedure in which the dielectric substrate was first sensitized by immersion in an acidic tin chloride solution followed by activation in a palladium chloride solution. Such catalytically activated surfaces would promote the generation of an electrolessly formed metal deposit layer through the oxidation of suitable components contained in an electroless plating bath. This initial electrolessly deposited conductive metal layer could be further plated through conventional electroplating. However, these two step catalyst systems had significant disadvantages.
For example, in the production of printed circuit boards having copper clad conductive surfaces on opposite sides of the insulating dielectric substrate it is a common practice to make electrical connections between the conductors on each side of the circuit board by forming holes through the board and then plating a conducting material on the surface of the "through holes" to interconnect the conducting layers. When utilizing the two step electroless deposition process to plate the surfaces of the through holes with a conducting metal it was found that the palladium chloride solution would produce an unwanted flash coating of palladium metal onto the copper clad surfaces of the substrate which subsequently had to be removed. As a result, this proved to be a wasteful and very expensive process.
Through holes are also utilized to interconnect the conducting layers of multi-layer laminated circuit boards having dozens of conductive layers separated by non-conducting substrate materials. Unfortunately, the acidic catalyst solutions utilized for electroless plating would also attack the black copper oxide layers of the multi-layer substrates, creating cavities or "voids" between the copper layer and the non-conducting layers which could become sites for chemical contamination and corrosion, thus interfering with the conductivity of the layers.
Improvements in the catalytic process involved the formation of a mixed palladium/tin catalytic system which combined the sensitization and activation steps in a single solution. An exemplary system is disclosed in U.S. Pat. No. 3,011,920 (Shipley). These single step catalytic baths were formed of aqueous solutions containing high concentrations of HCl in which varying amounts of palladium chloride and tin chloride were dissolved to produce colloidal suspensions. Though an improvement over the earlier two step catalytic processes, these combined catalytic solutions were subject to instability and indiscriminate deposition of metal onto the walls of plating bath containers and therefore required careful monitoring and control. In spite of these drawbacks, electroless deposition of an initial layer of metal has been an integral part of virtually all processes used for metalizing non-metallic surfaces.
A subsequent attempt to avoid the substantial shortcomings of electroless deposition procedures was disclosed in U.S. Pat. No. 3,099,608 (Radovsky, et al). This process attempted to avoid the need for an electroless plating step by directly electroplating a metal onto a thin, non-conducting layer of semi-colloidal palladium. This non-conductive palladium film was deposited onto the substrate surface utilizing the same palladium/tin catalyst solution used for electroless metal deposition. Though the palladium film layer was non-conductive (resistance was measured at approximately 8.times.10.sup.7 ohms per through hole) it was reportedly possible to electroplate over the film. However, deposition of the electroplated metal only began at the interface of a conductive surface and the non-conductive palladium catalyst film layer. Thus, propagation of the electroplated metal layer would grow epitaxially along the palladium layer from this interface and, as a result, direct metal deposition onto this layer was a very slow process. Moreover, it was often necessary to attach a conducting metal plating clip to the substrate in order to initiate electrodeposition at the interface between the clip and the substrate. Without a plating clip this process was limited to electroplating non-conductive substrates in areas in close proximity to a conductive surface. An additional drawback was the requirement of relatively large through holes in order to insure sufficient plating to constitute an effective connector.
An alternative approach to direct plating was disclosed in United Kingdom Patent Application No. GB2,123,036A (Morrissey, et al). This process provided a method for electroplating nonmetallic surfaces without the need for the prior electroless deposition of conducting metal through the use of preferential plating bath additives. As with the earlier methodologies, the non-conducting surface to be plated was first treated with a catalytic palladium/tin solution to form metallic sites on the surface. Electroplating was then accomplished through the utilization of preferential additives in the electroplating solution which were reported to inhibit the deposition of metal onto metal surfaces formed by plating without inhibiting deposition on the catalytically formed metallic sites over the non-conductive surface. As with the previously described direct plating process of U.S. Pat. No. 3,099,608, this method also required the presence of a conductive surface adjacent to the metallic sites for the initiation and propagation of the electroplated metal deposit.
One disadvantage of this methodology was the fact that it was limited to plating relatively large through holes due to the current density requirements of the process. More specifically, the electroplating current density commonly used with this procedure was on the order of 3 to 6 A/dm.sup.2. Because the circuit density required to initiate the plating process could not be substantially decreased, decreasing the diameter of the through holes resulted in their becoming filled with metal, thereby limiting the applicability of the process. A further disadvantage of this process was that the electrically deposited metal would plate over the entire conductive surface of cladded substrates in addition to the treated through hole walls. This resulted in the deposit and thus the subsequent removal of additional plating metal, undesirably contributing to the expense of the process.
A further limitation to this process is its inapplicability to modern pattern plating circuit board construction techniques. In pattern plating processes the electrodeposition of metal does not take place until after the substrate has been imaged with a photo resist to form a circuit pattern. As a result, electroplated metal will not cover the entire surface of the substrate. Unfortunately, treatment of the conductive cladding of the substrate prior to application and development of the photo resist utilizes chemicals found to dissolve or desorb the discrete metallic sites previously deposited on the through hole walls. As a result, directly electroplating the through holes is rendered impossible, making the methodology inappropriate for use with pattern plating.
An alternative approach to direct plating was disclosed in European Patent No. 0,298,298A2 (Bladon). This process provided for the electroplating of a non-conducting surface after a conversion treatment which converted an adsorbed colloid surface coating into a chemically resistant "conversion coating" which could function as a base for direct electroplating. A similar process was disclosed in U.S. Pat. No. 4,810,333 (Gulla, et al). Though conversion coatings will withstand the chemical treatments found in pattern plating techniques, these processes are as complex as electroless plating and utilize hazardous chemicals which are difficult and expensive to dispose of. An additional disadvantage of conversion coatings is the relatively high current density required to achieve direct electroplating which limits these processes to plating large through holes.
Accordingly, it is a principle object of the present invention to provide a method for directly electroplating a metal onto the surface of a non-conductive substrate. It is a further object of the present invention to provide a method for directly electroplating a metal layer onto the surface of a non-conducting substrate without the need for conversion treatments or electroplating solution additives.
It is a further additional object of the present invention to provide a method for the direct electroplating of non-conducting substrates that is particularly well suited to utilization in pattern plating processes. Moreover, it is an object of the present invention to provide a process for directly electroplating the surfaces of non-conducting substrates without the need for high plating current densities and correspondingly large through hole diameters.
It is an additional object of the present invention to provide treated substrates and directly electroplated dielectric substrate circuit boards that are particularly well suited for higher circuit density constructions, molded circuits, and multi-layer circuit board applications. It is a further object of the present invention to provide directly electroplated dielectric substrate circuit boards and molded circuits that are pattern plated.